Conventional multicolor electrostatic imaging systems utilize a separate toner bath to develop each desired color. This use of separate toner baths is relatively expensive from the standpoint of equipment complexity, cost, maintenance, and processing time expended. It also requires multiple mechanical registrations to produce the multicolor image--a requirement fraught with the potential for error.
As an alternative to the use of toners and electrostatic imaging, a recent development in the industry utilizes an imaging sheet of paper completely coated on one side with microencapsulated color precursors. A portion of the microcapsules on the sheet is selectively hardened by exposure to light. The microcapsules having the desired color precursor in the image areas have liquid cores which remain unhardened. These unhardened microcapsules are then ruptured to release liquid color precursor. The thus-released color precursor is contacted with a color developer to provide the color image, generally by transfer to a developer sheet via pressure contact of the imaging sheet with the developer sheet. Alternately, the color precursor-containing capsules are coated directly on a layer of developer material, which itself had previously been coated on a paper support.
By way of illustration, such a transfer imaging system containing microencapsulated color precursors is disclosed in U.S. Pat. No. 4,554,235, assigned to Mead Corporation. In a variation of this type of system, U.S. Pat. No. 4,501,809, assigned to Mitsubishi Paper Company, discloses a recording sheet containing two different types of photo- and pressure-sensitive microcapsules--one set containing color precursors and the other set containing color developer. Upon rupture of unhardened microcapsules on the recording sheet after selective exposure of the recording sheet to light in imagewise registration with an image to be copied, a color image is formed on the recording sheet.
The color imaging systems illustrated by the above-cited patents possess a common disadvantage. Both systems utilize an imaging or developer sheet containing microcapsules across a full surface of the sheet. Since in many color imaging applications the desired color image rarely occupies the full sheet, and, indeed, often occupies less than half of the full sheet, there is a significant amount of waste attributable to the unused microcapsules and associated color precursor or developer contained on the non-imaged areas of the sheet. In addition, there is a substantial time and energy waste attributable to the need for photohardening the "unused" waste microcapsules using, for example, a scanning laser.
In view of the above, new systems for multicolor imaging utilizing microcapsules which do not result in such substantial waste of microcapsules and the associated colorant materials, plus wasted time and energy due to the need for photohardening of the waste microcapsules, would be highly desired by the color imaging community.